Semiconductor wafers are typically fabricated with multiple copies of a desired integrated circuit design that will later be separated and made into individual chips. A common technique for forming the circuitry on a semiconductor wafer is photolithography. Part of the photolithography process requires that a special camera focus on the wafer to project an image of the circuit on the wafer. The ability of the camera to focus on the surface of the wafer is often adversely affected by inconsistencies, unevenness, or scratches on the wafer surface. This sensitivity is accentuated with the current drive for smaller, more highly integrated circuit designs which cannot tolerate certain nonuniformities within a particular die or between a plurality of dies on a wafer. Because semiconductor circuits on wafers are commonly constructed in layers, where a portion of a circuit is created on a first layer and conductive vias connect it to a portion of the circuit on the next layer, each layer can add or create nonuniformity on the wafer that must be smoothed out before generating the next layer.
Chemical mechanical planarization (CMP) techniques are used to planarize the raw wafer and each layer of material added thereafter. Available CMP systems, commonly called wafer polishers, often use a rotating wafer holder that brings the wafer into contact with a polishing pad moving in the plane of the wafer surface to be planarized. The polishing pad used in the CMP process is typically a disk or a belt. In some systems, a polishing fluid, such as a chemical polishing agent or a slurry containing microabrasives, is employed. The wafer holder then presses the wafer against the rotating polishing pad and is rotated to polish and planarize the wafer in order to create a smooth surface and remove any nonuniformities. The surface of the wafer is often completely covered by, and in contact with, the polishing pad to simultaneously polish the entire wafer surface.
During the planarization process of a wafer, scratches on the surface of the wafer being polished can result. These scratches may be caused by agglomeration of particles suspended in the polishing fluid. These agglomerates can comprise several small particles that are tightly bound together to form larger particles. As such, these large particles can be reduced in size through mechanisms capable of separating the small particles in a manner in which they can be dispersed throughout the polishing fluid.
Agglomerates of particles can form as a result of microabrasives that are suspended in a polishing fluid that do not become distributed throughout the polishing fluid. These microabrasives can be formed when the polishing fluid is manufactured such that the particles are not thoroughly dispersed throughout the polishing fluid. Alternatively, the agglomerates may form during the application of the polishing fluid from a polishing fluid dispensing mechanism of a CMP apparatus. The formation of the agglomerates may occur through a variety of different methods or result from particular chemicals or processes leading up to, or including, the planarization of a semiconductor wafer. A disadvantage of the presence of the agglomerates in the polishing fluid during planarization is that these large particles can cause scratches on the surface of the wafer. The scratches produced by the agglomerates during planarization of a wafer can damage, or even destroy, features on a wafer, thereby rendering sections or all of a wafer useless.